It has to do with the pronunciation. If changing the last letter would affect the pronunciation of the previous letter, that is "c" or "g", an h is added to keep the pronunciation consistant. So: la mucca - le mucche, l'amica - le amiche. There are, however, some exceptions, such as l'amico - gli amici.
Sorry I'm going to say you :/ The first translation was telling you that by "larghe" it meant loose but in terms of how it fits and not loose in some other way (so it probably won't work for hair that is loose). Notice when you hover over the word again that "larghe" is in black font and the "fitting" follows after a dash, in grey, and in parentheses. Just writing "loose" would have been correct.